Mobile devices have become ubiquitous in today's world and are increasingly used to access various communication services (e.g., voice calls, video calls, messaging, streaming multimedia content, playing high definition online games, and so forth) over wireless communication networks. A wireless communications network may include a number of base stations (BS's), each supporting communication for a number of mobile devices or user equipment (UE's). A UE may communicate with a BS during downlink and uplink, using various transmission protocols. The downlink (or forward link) refers to the communication link from the BS to the UE, and the uplink (or reverse link) refers to the communication link from the UE to the BS. Further, the wireless communication networks may correspond to multiple-access networks capable of supporting multiple users (i.e., UE's) by sharing the available network resources (e.g., time, frequency, and power). For example, conventional third generation (3G) and fourth generation (4G) wireless communication networks employ various multiple access techniques, such as code division multiple access (CDMA) in 3G, and frequency division multiple access (FDMA) or time division multiple access (TDMA) in 4G.
A long term evolution (LTE) network is a 4G wireless communication network, and is an end to end Internet protocol (IP) network supporting only packet switching. LTE network provides for high sector capacity, improved end-user throughputs, and reduced user plane latency. It therefore provides for significantly improved user experience along with greater mobility. The LTE network includes a number of 4G enable UE's, a number of evolved Node B's (eNB's) as base stations, and an evolved packet core (ePC). The user's application data (UAD) are transmitted over Ethernet channels between the ePC and the eNB's, and over air interface between the eNB's and the UE's. The data packets are transmitted between the UE's and the eNB's in downlink as well as in uplink using a data packet transmission protocol known as packet data convergence protocol (PDCP), as well as using various other protocols such as radio link control (RLC) protocol, medium access control (MAC) protocol, and so forth. For example, the downlink (DL) data packets flow through the PDCP, RLC and MAC protocol handlers within the eNB while the uplink (UL) data packets flow through the PDCP, RLC and MAC protocol handlers within the UE.
The DL data packets is received at the PDCP handler in the eNB from the ePC (i.e., from a signaling gateway (SGW) in the ePC through a gateway tunneling protocol (GTP-U)). The PDCP handler stores these data packet in PDCP transmission buffers (PTB's), and then sends them to the RLC handler after processing the data packets, if configured, for integrity protection and for ciphering. The received data packets are maintained in a first in first out (FIFO) queue in RLC transmit buffers (RTB's) at the RLC handler. The RLC handler then informs the MAC handler regarding its transmission buffer size by sending an internal control message (ICM) such as MAC status request. In turn, the MAC handler provides the RLC handler with its current available capacity information by sending transmission opportunity message (MAC status indication). The RLC handler then compiles the data packets from one or more buffers (queues) based on the transmission opportunity information. The compilation may involve concatenation and/or segmentation along with header addition. The RLC handler then sends the compiled message as MAC data request to the MAC handler by putting the compiled message into the MAC queue. Finally, the MAC handler processes the data packets and send it to radio subsystem of the ENB for air transmission.
It should be noted that each radio bearer (RB) has its own quality of service (QoS) requirements (e.g. time delay budget is one such QoS requirement). Due to such requirement, if the received data packets in DL at the eNB can't be transmitted to the UE in a reasonable time frame, it may be too late for a receiver application at the UE to accept the incoming data packets. In such a scenario, retransmission may occur at an application level. Thus, the PDCP handler in the eNB starts a timer for each received packet in DL so as to maintain the time delay budget for each data packet in the RB. This timer is known as a PDCP discard timer (PDT).
Thus, whenever data packets arrive at the PDCP handler from the SGW via GTP-U, the PDCP handler stores the data packets in the PTB and starts a PDT with expiry value depending on factors like user specific QoS and/or service specific QoS, for each received packet. The PDCP handler then sends the data packets to the RLC handler after making the data packets integrity protected and ciphered, if the PDCP handler is so configured. On expiry of the PDT, the PDCP handler discards or clears the corresponding data packet from the PTB, and sends the ICM to indicate the RLC handler that the particular packet is deleted from the PTB, and there is no need to transmit the data packet. Additionally the PDCP handler informs the RLC about the discarded packet sequence number through the ICM. The RLC then deletes corresponding packet from its buffer if it is not transmitted already. As will be appreciated, a similar operation may be performed at the UE during uplink.
However, incoming data rate at the PDCP handler may be variable for different users based on service usage and the corresponding QoS. Additionally, for downlink, incoming data rate at the PDCP handler may also depend on the number of active users under the coverage area of the eNB. Thus, operational load of UE (UOL) or operational load of eNB (EOL) varies dynamically. For example, for each received packet and on expiry of PDT, sending ICM to indicate the RLC handler for discarding of that particular packet adds to significant processing overhead. The processing overhead further compounds as there may be possibility of accumulation of too many timer expiry at a given moment pending packet discard. The processing of such pending packet discard may take longer, where further PDT's may get expired adding to the pending list of packet discard.
The PDCP handler may get clogged by the pending packet discard waiting to happen. This may result in the PDCP and the RLC buffer overflow. This may further prevent the PDCP handler to accept the newly arrived data packets due to buffer overflow. The overhead of accumulated packet discard at the PDCP handler, eventually leads to delay in handling packets discards for the subsequent PDT expiry. If the delay is larger than the delay budget for a data packet, then it may be possible that the data packet may be transmitted beyond an acceptable time-window (i.e., delay budget). Thus, it may be too late for a receiver application at the UE or a receiver application at the eNB to accept the incoming packet. In a typical eNB or UE, the PDCP buffer length is high. In some cases, at full load, the number of active PDT's for all pending discard-packets may be equal to the length of the buffer. Also, sending ICM to the RLC handler for each of these discarded packets is process and resource intensive. This may further add to processing overhead for the eNB or the UE. All such scenarios may lead to degradation of the service quality at the UE. Also, transferring packets too late to the UE or the eNB may be unnecessary, leading to bandwidth waste on air interface and packet discard overhead for the eNB or the UE.
Current techniques try to address the processing overhead issues for the data packet transmission in the eNB by reducing the ICM (e.g MAC-Status-Request) between PDCP handler and other downlink handlers (for example RLC & MAC). However, current techniques fail to provide handling of large number PDTs and their expiry to address the processing overload issue. Further, cloud radio access network (C-RAN) comprises of multiple eNB's. A centralized baseband unit (C-BBU) of C-RAN represent the base band part of these constituent eNB's. In the context of user packet transmission (i.e., data packet transmission in downlink) in case of CBBU, if there is a collapsed PDCP and a collapsed RLC, the above mentioned issues are further aggravated.